John J. McMahon was born in Hartford on April 22, 1875. He served as a volunteer in the First Regiment Infantry Connecticut National Guard, the Spanish-American War, and retired as a Colonel from the Connecticut National Guard on November 23, 1931. He was an officer and member in the First Connecticut Volunteer Infantry Veteran Association, Spanish-American War - 1898 and the Department of Connecticut United Spanish War Veterans. McMahon was elected to the Hartford Board of Education from 1911-1916, elected City Alderman from Hartford's seventh ward from 1922-1926, and worked as an architect in Hartford from 1893-1958.
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